One of the factors closely related to the clearance of solutes during HD is the blood flow; in order for the blood flow to be adequate, it is important to consider the needle calibre. We studied the effect of changes in needle calibre on the efficiency of HD, considering the type of vascular access. A prospective study in 66 HD patients was designed (48 with native fistula and 18 with con prosthetic), in which the needle calibre was modified. Pain, bleeding, coagulation times, vein pressure and Kt/V were recorded. The use of 15G needs in the arterial puncture improves the clearance of urea in native fistulas but not in the prosthetic fistulas. The use of 14G needles causes more episodes of pain, bleeding around the puncture site and prolongation of the coagulation time, and therefore we recommend the use of 15G needles to achieve greater efficiency in HD at flows of 350 ml/min.